The present invention generally relates to the preparation of semiconductor material substrates, especially silicon wafers, which are used in the manufacture of electronic components. More particularly, the present invention relates to a process for the thermal treatment of silicon wafers to dissolve agglomerated vacancy defects and improve internal gettering capabilities, enabling such wafers to form ideal, non-uniform depth distributions of oxygen precipitates during the heat treatment cycles of essentially any arbitrary electronic device manufacturing process.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared with the so-called Czochralski process wherein a single seed crystal is immersed into molten silicon and then grown by slow extraction. As molten silicon is contained in a quartz crucible, it is contaminated with various impurities, among which is mainly oxygen. At the temperature of the silicon molten mass, oxygen comes into the crystal lattice until it reaches a concentration determined by the solubility of oxygen in silicon at the temperature of the molten mass and by the actual segregation coefficient of oxygen in solidified silicon. Such concentrations are greater than the solubility of oxygen in solid silicon at the temperatures typical employed in fabrication processes for electronic devices. As the crystal grows from the molten mass and cools, therefore, the solubility of oxygen in it decreases rapidly, whereby in the resulting slices or wafers oxygen is present in supersaturated concentrations.
In addition to oxygen, intrinsic point defects, such as silicon lattice vacancies, may be present in the crystal as it forms. Like oxygen, the solubility of vacancies in the solid silicon is in part temperature dependent. As the silicon crystal cools, at some point the crystal may become critically supersaturated with vacancies, resulting in the formation of agglomerated vacancy defects. Agglomerated vacancy defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, and crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. Agglomerated vacancy defects are recognized to be a hindrance to the performance of electronic devices manufactured from wafers containing them.
The thermal treatment cycles typically employed in the fabrication of electronic devices can cause the precipitation of oxygen in silicon wafers which are supersaturated in oxygen. Like the agglomeration of vacancy defects, the precipitation of oxygen may also act to hinder device performance, depending upon the location within the wafer at which precipitation occurs. For example, oxygen precipitates located in the active device region of the wafer can impair the operation of the device. In contrast, oxygen precipitates located in the bulk of the wafer are advantageous because they are capable of trapping undesired metal impurities that may come into contact with the wafer. The use of oxygen precipitates located in the bulk of the wafer to trap metals is commonly referred to as internal or intrinsic gettering (xe2x80x9cIGxe2x80x9d).
A number of approaches have been suggested as to how agglomerated defects may be prevented from forming, or eliminated once formed. For example, the dissolution or annihilation of agglomerated intrinsic point defects may generally be achieved using high temperature heat treatments of the silicon in wafer form. (See, e.g., Fusegawa et al., European Patent Application 503,816 A1 and S. Nadahara et al., xe2x80x9cHydrogen Annealed Silicon Wafer,xe2x80x9d Solid State Phenomena, vols. 57-58, pp. 19-26 (1997).) However, while these approaches may have the desirable effect of dissolving agglomerated defects in the near surface region of the wafer, they may also result in the dissolution of oxygen precipitates in the bulk of the wafer, causing internal gettering capabilities to be lost.
Accordingly, a need continues to exist for a process wherein agglomerated intrinsic point defects may be dissolved or annihilated from a silicon wafer, and yet still afford a wafer possessing the beneficial characteristics of internal gettering.
Among the objects of the invention, therefore, is the provision of a single crystal silicon wafer which, during the heat treatment cycles of essentially any electronic device manufacturing process, will form an ideal, non-uniform depth distribution of oxygen precipitates; the provision of such a wafer which will optimally and reproducibly form a denuded zone of sufficient depth and a sufficient density of oxygen precipitates in the wafer bulk; the provision of such a wafer in which the formation of the denuded zone and the formation of the oxygen precipitates in the wafer bulk is not dependant upon differences in oxygen concentration in these regions of the wafer; the provision of such a wafer in which the thickness of the resulting denuded zone is essentially independent of the details of the IC manufacturing process sequence; the provision of such a wafer in which the formation of the denuded zone and the formation of the oxygen precipitates in the wafer bulk is not influenced by the thermal history and the oxygen concentration of the Czochralski-grown, single crystal silicon ingot from which the silicon wafer is sliced; the provision of such a process in which the formation of the denuded zone does not depend upon the out-diffusion of oxygen; and, the provision of such a process in which the concentration of agglomerated vacancy defects is substantially reduced in a surface layer of the wafer.
Briefly, therefore, the present invention is directed to a single crystal silicon wafer having two major, generally parallel surfaces, one of which is the front surface of the wafer and the other of which is the back surface of the wafer, a central plane between the front and back surfaces, a circumferential edge joining the front and back surfaces, a stratum which comprises the region of the wafer between the front surface and a distance, Ds, measured from the front surface and toward the central plane, a surface layer which is, at least in part, coextensive with the stratum and which comprises the region of the wafer between the front surface and a distance, Dl, of at least about 10 micrometers measured from the front surface and toward the central plane, and a bulk layer which comprises the region of the wafer between the central plane and the surface layer. The wafer is characterized in that the stratum is substantially free of agglomerated vacancy defects. Additionally, the wafer has a non-uniform distribution of crystal lattice vacancies with the concentration of vacancies in the bulk layer being greater than the concentration of vacancies in the surface layer with the vacancies having a concentration profile in which the peak density of the vacancies is at or near the central plane with the concentration generally decreasing from the position of peak density in the direction of the front surface of the wafer.
The present invention is further directed to a process for heat-treating a single crystal silicon wafer to dissolve agglomerated vacancy defects and to influence the precipitation behavior of oxygen in the wafer in a subsequent thermal processing step, the silicon wafer having two major, generally parallel surfaces, one of which is the front surface of the wafer and the other of which is the back surface of the wafer, a central plane between the front and back surfaces, a circumferential edge joining the front and back surfaces, a stratum extending from the front surface to a distance, Ds, as measured from the front surface and toward the central plane, a surface layer which, at least in part is coextensive with the stratum and which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, and a bulk layer which comprises the region of the wafer between the central plane and the surface layer. The process comprises thermally annealing the wafer in an atmosphere to dissolve pre-existing agglomerated vacancy defects in the stratum, heat-treating the annealed wafer to form crystal lattice vacancies in the surface and bulk layers, and then controlling the cooling rate of the heat-treated wafer to produce a wafer having a vacancy concentration profile in which the peak density is at or near the central plane with the concentration generally decreasing in the direction of the front surface of the wafer, the difference in the concentration of vacancies in the surface and bulk layers being such that a thermal treatment of the wafer at a temperature in excess of 750xc2x0 C. is capable of forming a denuded zone in the surface layer and oxygen clusters or precipitates in the bulk layer with the concentration of the oxygen clusters or precipitates in the bulk layer being primarily dependant upon the concentration of vacancies.
The present invention is further directed to a process for heat-treating a single crystal silicon wafer to dissolve agglomerated vacancy defects and to influence the precipitation behavior of oxygen in the wafer in a subsequent thermal processing step, the silicon wafer having a front surface, a back surface, a central plane between the front and back surfaces, a stratum extending from the front surface to a distance, Ds, as measured from the front surface and toward the central plane, a surface layer which at least in part is coextensive with the stratum and which comprises the region of the wafer between the front surface and a distance, D, measured from the front surface and toward the central plane, and a bulk layer which comprises the region of the wafer between the central plane and surface layer. The process comprises heat-treating the wafer in an atmosphere to form crystal lattice vacancies in the surface and bulk layers, and then controlling the cooling rate of the heat-treated wafer to produce a wafer having a vacancy concentration profile in which the peak density is at or near the central plane with the concentration generally decreasing in the direction of the front surface of the wafer and the difference in the concentration of vacancies in the surface and bulk layers being such that a thermal treatment of the wafer at a temperature in excess of 750xc2x0 C. is capable of forming a denuded zone in the surface layer and oxygen clusters or precipitates in the bulk layer with the concentration of the oxygen clusters or precipitates in the bulk layer being primarily dependent upon the concentration of vacancies. Oxygen precipitate nucleation centers present in the cooled wafer are stabilized by heating the cooled wafer to a temperature of about 650 to about 850xc2x0 C. for about 1 hour to about 4 hours, and then the stabilized wafer is thermally annealed in an atmosphere to dissolve agglomerated vacancy defects present within a region extending from the front surface to a depth of at least about 5 microns.